The “brake-by-wire” type brake system for a motor vehicle, illustrated in FIG. 1, is known from the prior art and is composed essentially of a brake activation unit 10, a brake pedal 10, a pedal travel simulator 2, an electronic control unit 7, which is illustrated only schematically, and wheel brakes (not shown) which are connected to the brake activation unit 10 with the optional intermediate connection of a hydraulic open-loop or closed-loop control unit. The brake activation unit 10 is formed by a brake booster, preferably an underpressure brake booster 3, a master brake cylinder, for example a tandem master cylinder 4, which is connected downstream of the brake booster 3 and to whose pressure spaces (not illustrated) the abovementioned wheel brakes of the motor vehicle are connected, as well as a pressure medium reservoir vessel 5 which is assigned to the master brake cylinder 4. The brake pedal 1, which serves for the activation of the brake booster 3 by the driver, interacts with the pedal travel simulator 2, in particular in the “brake-by-wire” operating mode, which pedal travel simulator 2 gives the driver the desired brake pedal sensation. A sensor device 6, which is preferably of redundant design, for detecting a driver's deceleration request generates, as a function of the activation of the brake pedal 1, control signals which are fed to the electronic control unit 7, by means of the output signals of which control unit 7 it is possible, inter alia, to actuate an electromagnet (not illustrated) which is assigned to the brake booster 3 and which permits a pneumatic control valve to be activated independently of the driver's will, which pneumatic control valve controls an air supply to the brake booster 3. An axial gap which is provided between the end of a piston rod 8, coupled to the brake pedal 1, and a control piston of the abovementioned control valve ensures, in the “brake-by-wire” operating mode, decoupling of the force transmitting connection between the brake pedal 1 and the brake booster 3.
The pedal travel simulator 2, by means of which, as already mentioned, a restoring force which acts on the brake pedal is simulated in the “brake-by-wire” operating mode independently of activation of the brake booster 3, is embodied in such a way that in the “brake-by-wire” operating mode said pedal travel simulator 2 can be activated when the force transmitting connection between the brake pedal 1 and the brake booster 3 is decoupled, and can be deactivated outside the “brake-by-wire” operating mode. The activation and deactivation of the pedal travel simulator 2 are carried out by an electro-hydraulic device 9, which is formed essentially by a hydraulic cylinder-piston arrangement 11, 12 which can be shut off by means of a check valve 14 which can be activated electro-magnetically. The cylinder-piston arrangement 11, 12 has a hydraulic pressure space 12 which is bounded by a piston 11 and a hydraulic low pressure chamber 13 which is connected to the pressure space 12, wherein the check valve 14 permits the shutting off and the opening of the connection. The check valve 14, which can be activated or switched over by means of the actuation signals of the electronic control unit 7, is embodied as a 2/2-way valve which can be activated electro-magnetically and is open in the currentless state (SO). A nonreturn valve, which is open toward the pressure space 12 and which is not illustrated in FIG. 1, serves to equalize the pressure between the pressure space 12 and the low pressure chamber 13.
In the arrangement which is previously known from the prior art, the pedal travel simulator has a spring element 16 which is provided as a compression spring and which is clamped between the brake pedal 1 or a force transmitting part 17, which is connected in a rotationally fixed fashion to the brake pedal 1, and a two-armed lever 18. The two-armed lever 18 is mounted opposite the brake pedal 1, offset coaxially with respect thereto in such a way as to be rotatable to a limited degree, wherein the first arm 19 of said two-armed lever 18 forms a support face for the spring element 16, while the second arm 20 thereof is supported on the hydraulic piston 11 of the piston-cylinder arrangement 9.
The force transmitting part 17 interacts with an elastic simulator element 15, preferably manufactured from rubber, for the brake pedal 1. In order to generate a hysteresis which, during the rising stroke of the pedal travel simulator 2, generates frictional forces in addition to the force of the spring element 16, which frictional forces counteract the activation force acting on the brake pedal 1, a friction element 21 bears against the force transmitting part 17 under the effect of the spring element 16. The friction element 21 interacts with a friction face 22. The friction element 21 bears against the force transmitting part 17 by means of obliquely arranged bearing faces 23, 24, in such a way that during the activation of the pedal travel simulator 2 a force component is produced which presses the friction element 21 against the friction face 22. The abovementioned elements 16, 17, 21, 22 are preferably arranged in a housing 25 which is preferably embodied in one piece with the first arm 19 of the two-armed lever 18.
The method of functioning of the brake system described above is known to a person skilled in the art who is active in the technical field of “brake-by-wire” systems. In the position of rest of the brake system (illustrated in FIG. 1) which also corresponds to the fallback level, the hydraulic pressure space 12 of the electro-hydraulic device 9 is connected to the low pressure chamber 13 via the check valve 14 which is open in the currentless state. During the activation of the brake pedal 1, the movement thereof is detected by the sensor device 6 and signaled to the electronic control unit 7, which at the same time generates control signals for actuating the abovementioned electromagnet and the check valve 14, with the result that the connection between the pressure space 12 and the low pressure chamber 13 is interrupted, causing the pedal travel simulator 2 to be activated. The check valve 14 is therefore closed in the “brake-by-wire” operating mode, with the result that neither the arm 20 nor the housing 25 of the pedal simulator can move. When the brake pedal 1 is activated by the vehicle driver, the spring element 16 is therefore compressed. Outside the “brake-by-wire” operating mode the valve 14 is opened, and when the brake pedal 1 is activated the simulator housing 25 rotates along with the brake pedal 1, as a result of which the force applied by the vehicle driver is not absorbed in the pedal travel simulator 2 but rather acts on the underpressure brake booster 3.
In the event of failure of the vehicle electronics or failure of the on-board power system, the check valve 14 cannot be switched over, with the result that the braking occurs in the fallback level. When the brake pedal 1 is activated, the two-armed lever 18 rotates along with the brake pedal 1 under the effect of the spring element 16, with the result that the piston 11 of the electro-hydraulic device 9 is pushed to the left in the drawing and the pressure medium is pushed out of the pressure space 12 into the low pressure chamber 13. The simulator 2 is deactivated.
The basic design and the method of functioning of the “brake-by-wire” brake system are known as is described in the prior art, with the result that they are not included in the description of the following exemplary embodiments of a brake system according to the invention and details are only given on the differences according to aspects of the invention.
A brake-by-wire system is known from DE 10 2004 011 622 A1, which is incorporated by reference, and is described, in particular, in conjunction with FIG. 15 in said document. The pedal travel simulator is accommodated in the previously known brake system by a housing in which a force transmitting element which is connected in a rotationally fixed fashion to the brake pedal projects. An electro-hydraulic device which can be actuated by means of the electronic control unit and which is formed by a piston-cylinder arrangement permits activation of the pedal travel simulator in the “brake-by-wire” operating mode and at least partial deactivation of the pedal travel simulator outside the “brake-by-wire” operating mode by virtue of the fact that the housing of the pedal travel simulator is supported on the electro-hydraulic device by means of a lever arm which is securely connected to the housing. It is considered disadvantageous in the previously known brake system that the function of the electro-hydraulic device has to be monitored. In addition, the known system is costly and has a high number of components.